Due to the development of compact handheld electronic devices, for example cell phone, MP3, PDA, iPod and LCD TV etc., towards to light, thin, short and small, highly efficient class-D amplifiers attract more and more attention. As shown in FIG. 1, a class-D amplifier 10 typically includes a ramp generator 14, a modulator 16, and a driver 18. The ramp generator 14 provides a ramp signal Vramp, and the modulator 16 converts an audio frequency input signal Vaudio into a pulse width modulation signal PWM according to the ramp signal Vramp, for the driver 18 to drive a load device 12. EMI performance is an important issue in class-D amplifiers, therefore many methods for improving EMI issue have been proposed, one of which is spread spectrum methodology.
Current spread spectrum methods, for example U.S. Patent Publication Nos. 2004/0232978 and 2007/0132509 and U.S. Pat. No. 6,847,257, use random circuit to provide a random number to change the frequency of the ramp signal Vramp and thereby generate an unpredictable PWM frequency to drive the audio amplifier. However, these arts require an extra random generator to provide a random number. Furthermore, a high audio fidelity requires a higher frequency ramp signal Vramp when the amplitude of the audio frequency input signal Vaudio is large, and requires a lower frequency ramp signal Vramp when the amplitude of the audio frequency input signal Vaudio is small. However, the random numbers provided by a random generator are unpredictable, and in consequence a lower frequency ramp signal Vramp may be generated when the amplitude of the audio frequency input signal Vaudio is large, thereby resulting in distortion.
Therefore, it is desired a distortion reduced class-D amplifier without a random generator.